Thousands rallied in the West Bank's two largest cities on Monday in support of four long-term hunger strikers, as the Palestinians demanded tougher EU action to help their prisoners.
The deteriorating health of prisoners on hunger strike in Israeli jails has sparked several mass protests across the Palestinian territories, some of which have sparked clashes with the army.
On Monday, more than 1,000 people joined a rally of support in the northern city of Nablus, while another 1,500 gathered in the centre of Hebron in the south to demand their release, AFP correspondents said.
With public anger growing over the fate of the prisoners, Palestinian negotiator Saeb Erakat sent a letter to EU foreign policy chief Catherine Ashton urging her to go beyond calls for Israel to improve its treatment of prisoners and to act to secure their release.
"We believe it is no longer acceptable to merely request better treatment of Palestinians in Israeli occupation prisons, but to demand an end to the arbitrary system of Israeli detentions," it said.
Flagging up the deteriorating health of the four prisoners -- Samer Issawi, Tareq Qaadan, Jafar Ezzedine and Ayman Sharawna -- Erakat said the Palestinians would hold Israel "fully responsible" if one of them died.
Over the weekend, Ashton issued a statement expressing concern over the deteriorating health of the four and urging Israel to respect its human rights obligations towards the prisoners and permit family visits.
But Erakat urged greater action.
"We are asking you to prevent a tragedy and take immediate and definitive action to secure the freedom of Palestinian prisoners on hunger strike," he told her.
He also urged the EU to make its strong trade relations with Israel dependent on the Jewish state's level of respect for international humanitarian law.
"We therefore urge you to consider the importance of human rights and international humanitarian law in your bilateral relations and agreements with the State of Israel."
The letter was sent a day ahead of a planned mass hunger strike by Palestinian prisoners in a move echoing a similar protest in April 2012 which ended with Israel easing their conditions.
Meanwhile in annexed east Jerusalem, Israeli police staged an overnight raid on Issawi's family home, arresting his brother Shaadi, the family and police said.
Police confirmed the arrest but refused to give further details.
On Tuesday, a Jerusalem court was to hold a hearing on Samer Issawi's case, his sister Shireen told AFP. She said he had not been due in court until March 14 and the family did not know what to expect.
According to the Addameer prisoner rights group, Qaadan, 40, and Ezzedine, 41, were both arrested in their home village of Araba in the northern West Bank on November 22 in a sweep which saw troops detaining 55 "terror operatives."
Both were handed a three-month detention order and began refusing food on November 28 to protest against their being held without charge under a procedure known as administrative detention.
Their detention orders are due to expire or be renewed on February 22.
Issawi, 33, and Sharawna, 36, were long-term security prisoners who were initially released by Israel under a prisoner swap deal in October 2011.
But within months, they were both rearrested following unspecified allegations that they violated the terms of the agreement, with Israel ordering them to serve out the remainder of their original sentences.
Sharawna was rearrested on January 31 and began refusing food on July 1 to protest against his rearrest and demand his immediate release.
Issawi was arrested on July 7 and stopped eating on August 1, to protest over his rearrest and retrial based on information which was not made available to him or his lawyer.
Statistics published by Israeli rights group B'Tselem show that at the end of 2012, 4,500 Palestinians were being held in Israeli jails.